It is an object of the present invention to provide an optical amplifying unit to be used for optical telecommunications. The invention also relates to an optical transmission system, more particularly a wavelength division multiplexing (WDM) optical transmission system, which uses the above-mentioned optical amplifying unit. The optical amplifying unit of the invention is also adapted to be used in analog applications, for example for CATV systems.
In WDM optical transmission systems, transmission signals including several optical channels are sent over a same line (that includes at least an optical amplifier) by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
Present-day long-distance high-capacity optical transmission systems use optical fiber amplifiers that, differently from previously used electronic regenerators, do not need OE/EO conversion. An optical fiber amplifier includes an optical fiber of preset length, having the core doped with one or more rare earths so as to amplify optical signals by stimulated emission when excited by pump radiation.
Optical fibers doped with erbium (Er) have been developed for use as both optical amplifiers and lasers. These devices are of considerable importance since their operating wavelength coincides with the third window for optical fiber communications, around 1550 nm.
Patent application EP 964275 in the name of the Applicant proposes a thirty-two channels WDM optical transmission system that uses erbium-doped fiber amplifiers (EDFAs) in the wavelength bands 1529-1535 nm and 1541-1561 nm.
Patent application EP 897205 A2 in the name of Fujitsu Limited describes a device comprising an erbium-doped fiber optical amplifier, and first and second optical filters operatively connected to the optical amplifier to suppress the wavelength dependence of gain in the gain bands 1.52-1.54 xcexcm and 1.54-1.58 xcexcm, the optical amplifier being pumped in a 0.98 xcexcm band or in a 1.48 xcexcm band.
Several methods have been proposed to improve the system performances in terms of amplification bandwidth in order to increase the number of channels to be transmitted. One way to increase channel numbers is to narrow the channel spacing. However, narrowing channels spacing worsens nonlinear effects such as cross-phase modulation or four wave mixing, and makes accurate wavelength control of the optical transmitters necessary. Applicant has observed that a channel spacing lower than 50 GHz is difficult to achieve in practice do to the above reasons.
Another way to increase the channel number is to widen the usable wavelength bandwidth in the low loss region of the fiber. One key technology is optical amplification in the wavelength region over the conventional 1550 nm transmission band. In particular, the high wavelength band around 1590 nm, and precisely between 1565 nm and 1620 nm, is a very promising band for long-distance optical transmissions, in that a very high number of channels can be allocated in that band. If the optical amplifier for the 1565-1620 nm band must deal with a high number of channels, the spectral gain characteristics of such amplifier are fundamental to optimize the system""s performances and costs. The use of the 1590 nm transmission wavelength region of erbium-doped fiber amplifiers in parallel to the 1530 and 1550 wavelength regions, is attractive and has been recently considered. As an additional advantage, by employing the 1590 nm wavelength region it is possible to use dispersion-shifted fiber (DSF) for WDM transmissions without any degradation caused by four-wave mixing.
Several articles in the patent and non-patent literature address amplification by erbium-doped fiber amplifiers in the high wavelength transmission band (from 1565 nm up to 1620 nm).
U.S. Pat. No. 5,500,764 relates to a SiO2xe2x80x94Al2O3xe2x80x94GeO2 single-mode optical fiber (having a length between 150 m and 200 m) doped with erbium, pumped by 1.55 xcexcm and 1.47 xcexcm optical sources and adapted to amplify optical signals between 1.57 xcexcm and 1.61 xcexcm.
Ono et al., xe2x80x9cGain-Flattened Er3+-Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 xcexcm Wavelength Regionxe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997, pp. 596-599, disclose a gain-flattened Er3+-doped silica-based fiber amplifier for the 1.58 xcexcm band WDM signal; different fiber lengths were tested and the authors found that 200 m was the optimum length of EDF (Erbium-Doped Fiber) for constructing an EDFA with high gain and low noise.
Masuda et al., xe2x80x9cWideband, gain-flattened, erbium-doped fibre amplifiers with 3 dB bandwidths of  greater than 50 nmxe2x80x9d, ELECTRONICS LETTERS, Jun. 5, 1997, Vol. 33, No. 12, pp. 1070-1072, propose a scheme with two-stage erbium-doped fibres and an intermediate equalizer, obtaining a 52 nm band (1556-1608 nm) for a silicate erbium-doped fiber amplifier and a 50 nm band (1554-1604 nm) for a fluoride erbium-doped fiber amplifier; in the case of a silicate erbium-doped fiber amplifier, the two stages include a 50 m EDF and a 26 m EDF, respectively. In the proposed experiment, the two EDFs are pumped by 1480 nm laser diode sources via dichroic mirror-type wavelength-selective couplers.
Jolley et al., xe2x80x9cDemonstration of low PMD and negligible multipath interference in an ultra flat broad band EDFA using a highly doped erbium fiberxe2x80x9d, xe2x80x9cOptical Amplifiers and their Applicationsxe2x80x9d Conference, Vail, Colo., Jul. 27-29 1998, TuD2-1/124-127 proposes a broad band EDFA which amplifies signals in the 1585 nm band using 45 m of erbium fiber and reaching a maximum external power of more than +18.3 dBm at 1570. The active fiber is bidirectionally pumped by a 980 nm laser diode and a 1480 nm laser diode.
The Applicant has observed that the pump wavelength is an important parameter for the design of optical amplifiers in the considered band, since it influences the amplifier""s performances in terms of gain efficiency and noise figure. This influence is not observable in conventional amplifiers in the 1550 nm band, in which the effects of the pump wavelength choice are prevalently on the spectral shape of the gain curve.
F. A. Flood and C. C. Wang, xe2x80x9c980-nm Pump-Band Wavelengths for Long-Wavelength-Band Erbium-Doped Fiber Amplifiersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11, No. 10, Oct, 1999, attests the importance of a careful choice of pump wavelengths to ensure optimum amplifier performance in the long-wavelength band (L-band) and shows the dependency of output signal power and backward amplified spontaneous emission (ASE) power on 980-nm band pump wavelength and input signal power for a L-band EDFA. In particular, this article demonstrates that tuning pump wavelength xc2x130-nm away from the 980-nm absorption peak provides 3-5 dB improvement in pump-to-signal conversion.
The Applicant has tackled the problem of providing an optical amplifier for the L-band with improved performances with respect to the known amplifiers above described.
The Applicant has first observed that 1480-nm pumping determines best performances with respect to 980-nm pumping in the considered amplifiers in terms of power conversion efficiency and quantum conversion efficiency. For power EDFA""s, the power conversion efficiency (PCE) can be defined as the ratio:   PCE  =                    P        s        out            -              P        s        in                    P      p      in      
where Psin, Psout and Ppin are the signal power at the input and at the output of the amplifier and the pump power at the input of the amplifier, respectively, while the quantum conversion efficiency (QCE) can be defined by:       QCE    =                                        φ            s            out                    -                      φ            s            in                                    φ          p          in                    =                                    λ            s                                λ            p                          ⁢        PCE              ,
where xcfx86pin, xcfx86sin and xcfx86sout are the input or output pump and signal photon fluxes (xcfx86p,sx=Pp,sx/hvp,s). The maximum possible value for the QCE is unity, which corresponds to the case where all pump photons are effectively converted into signal photons.
The Applicant has then found that, in the L-band of erbium-doped fiber amplifiers, for pump wavelengths below 1480 nm both the amplifier output power (and, then, the gain efficiency) and the noise figure are related to the pump wavelength used to provoke the population inversion of the active material. The Applicant has found that the phenomena here involved are different from those that operate in the case of pump wavelength lower than 980 nm.
The Applicant has in particular found that a very efficient and relatively low-noise optical amplifier adapted to operate in the L-band can be obtained by pumping an erbium-doped active fiber by means of one or more detuned 1480-nm pumps. The Applicant has further found that the performances of an optical amplifier adapted to operate in the L-band and pumped at 1480 nm or below 1480 nm can be improved by using active fibers having a numerical aperture and an aluminum concentration opportunely chosen within predetermined ranges.
Accordingly, the present invention relates, in a first aspect, to an optical transmission system including:
an optical transmitting unit to transmit optical signals in a transmission wavelength band above 1570 nm,
an optical receiving unit to receive said optical signals,
an optical fiber link optically coupling said transmitting unit to said receiving unit and adapted to convey said optical signals, and
at least an optical amplifying unit coupled along said link and adapted to amplify said optical signals; said optical amplifying unit having an amplification wavelength band including said transmission wavelength band and comprising:
an input for the input of said optical signals,
an output for the output of said optical signals,
at least an erbium-doped active fiber, having a first end optically coupled to said input and a second end optically coupled to said output, for the amplification of said optical signals,
a pump source for generating a pump radiation adapted to excite erbium, and
an optical coupler optically coupling said pump source to said at least an active fibre,
wherein said pump source has an emission wavelength greater than 1400 nm and lower than 1470 nm, preferably comprised between 1430 nm and 1460 nm.
Advantageously, said optical coupler mat be positioned between said input and the first end of said at least an active fiber to provide said pump radiation to said at least an active fiber in a co-propagating direction.
The amplifying unit may include a further pump source for generating a further pump radiation adapted to excite erbium, and a further optical coupler positioned between the second end of said at least an active fiber and said output to provide said further pump radiation to said at least an active fiber in a counter-propagating direction.
Preferably, said at least an active fiber includes a first and a second active fiber arranged in series.
Preferably, the optical transmission system is a WDM system.
Preferably, the width of said amplification wavelength band is at least 15 nm, more preferably at least 25 nm.
Preferably, said at least an active fiber has a core having a concentration of erbium between approximately 0.8xc2x71025 ions/m3 ppm and 1.6xc2x71025 ions/m3 ppm.
Preferably, said at least an active fiber has a total length lower than 70 m.
Preferably, said optical fiber link includes a plurality of optical fiber spans each having a length of at least 100 km.
Preferably, said pump source is a semiconductor laser diode.
Said amplification wavelength band may allocate at least 32 channels, more preferably at least 64 channels.
According to a second aspect, the present invention relates to a method for transmitting optical signals, comprising generating at optical signal having a wavelength greater than 1570 nm, transmitting said optical signal in a long-distance optical fiber link and receiving said optical signal, said step of transmitting including feeding said optical signal to at least an erbium-doped active fiber for amplification, wherein it includes providing to said at least an erbium-doped active fiber a pump radiation having a wavelength greater than 1400 nm and lower than 1470 nm.
Said pump radiation has a wavelength preferably greater than 1430 nm and lower than 1460 nm.
Said step of generating at optical signal may include generating a plurality of optical signals at respective wavelengths greater than 1570 nm, and said step of transmitting may include wavelength multiplexing said plurality of optical signals.
Said at least an erbium-doped active fiber may include a first and a second erbium-doped active fiber, and in said step of providing to said at least an erbium-doped active fiber a pump radiation may include providing to each of said first and second active fiber respective pump radiations in a co-propagating direction and/or in a counter-propagating direction.
According to a further aspect, the present invention relates to an optical amplifying unit having an amplification wavelength band above 1570 nm, comprising:
an input for the input of optical signals,
an output for the output of said optical signals,
at least an erbium-doped active fiber having a first end optically coupled to said input and a second end optically coupled to said output, for the amplification of said optical signals,
a pump source for generating a pump radiation adapted to excite erbium, and
an optical coupler optically coupling said pump source to said at least an active fibre,
wherein said pump source has an emission wavelength greater than 1400 nm and lower than 1470 nm.
Said at least an erbium-doped active fiber may include a first active fiber and a second active fiber arranged in series, said optical coupler feeding said pump radiation to said first active fiber, said optical amplifying unit including a further pump source for generating a further pump radiation adapted to excite erbium and a further optical coupler for feeding said further pump radiation to said second active fiber.
Said optical coupler may be positioned between said input and said first active fiber for feeding said pump radiation to said first active fiber in a co-propagating direction, and said further optical coupler may be positioned between said second active fiber and said output for feeding said further pump radiation to said second active fiber in a counter-propagating direction.
The active fiber has a core including a concentration of Al preferably comprised between 1% and 6% weight molar concentration, more preferably comprised between 2% and 3% weight molar concentration.
Said active fiber has a numeric aperture NA preferably comprised between 0.25 and 0.32, more preferably comprised between 0.27 and 0.3.
According to a further aspect, the present invention relates to an optical amplifying unit having an amplification wavelength band above 1570 nm, comprising:
an input for the input of optical signals,
an output for the output of said optical signals,
at least an active fiber having a core doped with erbium and aluminum and having a first end optically coupled to said input and a second end optically coupled to said output, for the amplification of said optical signals,
a pump source for generating a pump radiation at a wavelength between 1400 nm and 1480 nm for exciting erbium, and
an optical coupler optically coupling said pump source to said at least an active fibre,
wherein the Al concentration in the core of said active fiber is comprised between 1% and 6% weight molar concentration, preferably between 2% and 3% weight molar concentration.
Preferably, said active fiber has a numeric aperture NA comprised between 0.25 and 0.32, more preferably comprised between 0.27 and 0.3.
According to a further aspect, the present invention relates to an optical amplifying unit having an amplification wavelength band above 1570 nm, comprising:
an input for the input of optical signals,
an output for the output of said optical signals,
at least an active fiber having a core doped with erbium and aluminum and having a first end optically coupled to said input and a second end optically coupled to said output, for the amplification of said optical signals
a pump source for generating a pump radiation at a wavelength between 1400 nm and 1480 nm for exciting erbium, and
an optical coupler optically coupling said pump source to said at least an active fibre,
wherein the Al concentration in the core of said active fiber has a numeric aperture NA comprised between 0.25 and 0.32, preferably comprised between 0.27 and 0.3.
The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.